1. Field of the Invention
The present invention relates to the field of implantable biocompatible transducers, particularly those useful for a microactuator included in a fully implantable hearing aid system.
2. Description of the Prior Art
Patent Cooperation Treaty ("PCT") patent application Ser. No. PCT/US96/15087 filed Sep. 19, 1996, entitled "Implantable Hearing Aid" ("the 15087 PCT Patent Application") describes an implantable hearing aid system which uses a tiny implantable microactuator. A PCT patent application Ser. No. PCT/US97/02323 entitled "Improved Biocompatible Transducers" filed Feb. 14, 1997, ("the 02323 PCT Patent Application") discloses improved implantable microactuators that are useful in the fully implantable hearing aid system disclosed in the 15087 PCT Patent Application. The fully implantable hearing aid system disclosed in the 15087 and 02323 PCT Patent Applications can operate for a period of five years on a set of batteries, and produce sound levels of 110 dB. The fully implantable hearing aid system described in these PCT Patent Applications is extremely compact, sturdy, rugged, and provides significant progress towards addressing problems with presently available hearing aids.
As described in the 15087 PCT Patent Application, the microactuator is preferably implanted into a fenestration that pierces the promontory of the cochlea, and uses stress-biased lead lanthanum zirconia titanate ("PLZT") transducer material. Stress-biased PLZT materials exhibit very high deflections and generate very high forces in comparison with other existing piezoelectric materials and/or structures. Such materials provide in a monolithic structure both a layer of conventional PLZT and a compositionally reduced layer from which the PLZT oxide has been converted to an electrically conductive Crete material. During operation of the transducer, the PLZT layer expands and contracts laterally upon application of an alternating current ("AC") voltage across the disk. Expansion and contraction of the PLZT layer flexes the disk back-and-forth due to differential expansion between the PLZT layer and the unexpanding Crete layer. However, the Crete layer in that transducer material includes a metallic form of lead ("Pb") as one of its constituent elements.
Microactuators disclosed in the 15087 PCT Patent Application include a membrane diaphragm that provides good biological isolation for the transducer. Moreover, use of the membrane diaphragm fully preserves, or may, through the use of hydraulic amplification, actually enhance transducer performance by magnifying the transducer's deflection or displacement. Although the transducers disclosed in the 15087 PCT Patent Application usually attach the Crete layer to the membrane diaphragm and fully enclose that layer, a possibility still exists that lead may leach from the transducer.